1. Field of the Invention
This invention relates to a multi-function optical filter intended for multi-wavelength optical communication system and high density wavelength-division multiplexed (WDM) network systems. Besides the optical spectrum region, the invention described herein can also be applied to other electromagnetic spectra such as microwave, milliwave, etc.
2. Description of Prior Art
Optical filters are key devices for multi-wavelength optical communication systems and high density wavelength-division multiplexed (WDM) network systems. In order to maximize the present 30 nm communication window supported by an erbium fiber amplifier for WDM network systems, there is an increasing demand for new tunable optical filters that have the following requirements; (1) wide free spectral range (FSR&gt;30 nm), (2) narrow linewidth (.DELTA..lambda..sub.FWHM &lt;0.3 nm), (3) High SNR or unity contrast or visibility, (4) tunable and (5) fiber compatible.
Furthermore, for next generation, highly complex combination of optical TDM-WDM network systems, an additional requirement, namely (6) programmability or multi-function capability, would be necessary to make a compact, versatile, economical and most importantly intelligent network system. Toward this goal, most if not all key devices in the network system such as an optical filter, an add/drop multiplexer (ADM), etc. need to be programmable, at best, or need to possess multi-function capabilities.
At present, most optical filters used in optical communication fields are dedicated, single-function devices. These single-function optical filters can be divided into two classes depending on the physical mechanism used in the filtering process namely; (1) grating-based filter and (2) resonant-based filter. Examples of grating-based filters are fiber bragg-grating (FBG) filter [F. Bakhati and P. Sansonetti, "Design and realization of multiple quarter-wave phase shifts UV-written bandpass filters in optical fibers", J. Lightwave Technol. vol. 15, 1433-1437, 1997], acousto-opto tunable filter (AOTF) [J. Jackel, J. Baran, A. d'Alessandro, and D. Smith, "A passband-flattened acousto-optic filter", IEEE Photonics Tech. Letter, Vol. 7, 318-320, 1995] and array-waveguide grating filter (AWGF) [H. Takahashi, K. Oda, H. Toba, Y. Inoue, "Transmission Characteristics of Arrayed Waveguide N.times.N Wavelength Multiplexer", J. Lightwave Technol vol. 13, 447-455, 1995]. Examples for resonant-based filters are multilayer dielectric film [P. H. Lissberger and A. K. Roy, "Narrowband position-tuned multilayer interference filter for use in single-mode-fiber systems", Electr. Lett. Vol. 21. No. 18, 798-799, 1985], Mach-Zehnder interferometer (MZI) [M. Kuznetsov, "Cascaded Coupler Mach-Zehnder Channel Dropping Filter for Wavelength-Division Multiplexed Optical Systems", J. Lightwave Technol. vol. 12, 227-230, 1994 ], ring resonator [B. E. Little, S. T. Chu, H.A. Haus, J. Foresi, and J.-P. Laine, "Microring resonator Channel Dropping Filters", J. Lightwave Technol., vol. LT-15, pp. 998-1005, 1997], Fox-Smith resonator [P. Urquhart, "Compound optical-fiber-based resonator," J. Opt. Soc. Amer. A, vol. 5, pp. 803-812, 1988], and Fabry-Perot resonator [J. Stone and L. Stulz, "Pigtailed high-finesse tunable fiber Fabry-Perot interferometers with large, medium and small free spectral ranges", Elect. Lett. Vol. 23, 781-782, 1987].
From among these different types of optical filters, fiber ring resonator (RR) and fiber Fabry-Perot resonator (FPR) have the highest potential to realize the above first 5 requirements in a compact and stable configuration. Unfortunately, these filters lack programmability or multi-function capability. Optical filters with some programmability in principle like a cascaded optical fiber lattice are impractical to implement because of the numerous phases and coupling parameters that need to be controlled.